An organic light-emitting element emits light when electric field is applied thereto. The emission mechanism is a carrier injection type. That is, by applying voltage through a pair of electrodes that interposes an electroluminescent layer therebetween, electrons injected from a cathode and holes injected from an anode are recombined within the electroluminescent layer to form molecules in excited states (hereinafter, excited molecule), and the excited molecules return to the ground state while radiating energy to emit light.
There are two excited states possible from organic compounds, the singlet state and the triplet states. Light emission from the singlet state is referred to as fluorescence and the same from the triplet state is referred to as phosphorescence.
In such organic light-emitting element, an electroluminescent layer is generally formed to have a thickness of below 1 μm. Further, since the organic light-emitting element is a self-luminous element in which an electroluminescent layer emits light, a back light used for the conventional liquid crystal display device is unnecessary. Therefore, the organic light-emitting element has a great advantage of being manufactured to have a ultra thin film thickness and light weight.
In the case of an electroluminescent film with a thickness of approximately 100 nm to 200 nm, the time between the injection of carriers and their recombination is about several ten nanoseconds considering the carrier mobility. Hence, the time required for the process of injecting carriers and emitting light of the electroluminescent layer is on the order of microsecond. Thus, an extremely high response speed is one of the advantages thereof.
Further, since an organic light-emitting element is carrier injection type, it can be driven by a direct current voltage, thereby noise is hardly generated. With respect to a drive voltage, an electroluminescent layer is formed into a uniform ultra thin film having a thickness of approximately 100 nm, and a material for an electrode is selected to reduce a carrier injection barrier. Further, a hetero structure (two-layers structures is introduced. Accordingly, a sufficient luminance of 100 cd/m2 can be obtained at an applied voltage of 5.5 V (non-patent literature 1: C. W. Tang and S. A. VanSlyke, Applied Physics Letters, vol. 51, No. 12, pp. 913–915 (1987)).
An organic light-emitting element has been attracted attention as a next generation's device for a flat panel display in terms of the thin thickness and lightweight, the high response speed, the direct low voltage operation, or the like. In addition, the organic light-emitting element can be used effectively as the element for the display screen of a portable electric appliance in terms of the self luminous type, the wide viewing angle, and the high level of visibility.
Wide variations of emission color is also one of the advantages of the organic light-emitting element. Richness of color is resulted from the multiplicity of an organic compound itself. That is, an organic compound is flexible enough to be developed to various materials by designing molecules (such as introducing substituent). Accordingly, the organic light-emitting element is rich in color.
From these viewpoints, it would not be an overstatement to say that the biggest application area of an organic light-emitting element is a full color flat panel display device. Various means for full colorization have been developed in view of characteristics of the organic light-emitting element. At present, there are three primary methods of forming the structure of a full color light-emitting device by using the organic light-emitting element.
First, the method that organic light-emitting elements having three primary colors, that is, red (R), green (G), and blue (B) are patterned, respectively, by shadow mask technique to serve them as pixels (hereinafter, RGB method). Second, a blue organic light-emitting element is used as a light emission source, and the blue emission is converted into green or red by color changing material (CCM) made from phosphorescent material to obtain three primary colors (hereinafter, CCM method). Third, a white organic light-emitting element is used as a light emission source, and a color filter (CF) used for a liquid crystal display device or the like is provided to obtain three primary colors (hereinafter, CF method).
Among these, in the CCM system and the CF system, an organic light-emitting element that is used therein emits monochromatic light such as blue (CCF system) or white (CF system); accordingly, different from the RGB system, precise separate coating by use of a shadow mask is not necessary. Furthermore, a color conversion material or a color filter can be manufactured according to an existing photolithography technique, and there is no need of complicated processes. Still furthermore, other than merits from a process point of view, there is another advantage in that since only one kind of element is used, the brightness varies uniformly with time; accordingly, the color shift or irregular brightness with time are not caused.
However, in case of adopting the CCM method, there has been a problem in red color since color conversion efficiency of from blue to red is poor in principle. In addition, there has been a problem that the contrast becomes deteriorated since a color conversion material itself is fluorescent so that light is generated in pixels due to outside light such as sunlight. CF method has no such problems since a color filter is used as well as the conventional liquid display device.
Accordingly, although the CF method has comparative few disadvantages, the CF method has a problem that a high efficient white organic light-emitting element is indispensable to the CF method since a great deal of light is absorbed into the color filter. A mainstream white organic light-emitting element is the element that combines complementary colors (such as blue and yellow) (hereinafter, two wavelengths white light-emitting element) instead of white color having the peak intensity in each wavelength of R, G, and B (non-patent literature 2: Kido et al., “46th Applied Physics Relation Union Lecture Meeting” p1282, 28a-ZD-25 (1999)).
However, considering a light-emitting device combined with a color filter, a white organic light-emitting element having an emission spectrum with the peak intensity in each wavelength of R, G, and B (hereinafter, three wavelengths white light-emitting element) is desirable instead of the two wavelengths white light-emitting element, which was reported in the non-patent literature 2.
Such three wavelengths white light-emitting element has been reported several times (for instance, non-patent literature 3: J. Kido at al., Science, vol. 267, 1332–1334 (1995)). However, such three wavelengths white light-emitting element is inferior to the two wavelengths white light-emitting element in terms of luminous efficiency, consequently, significant improvement is required.
Furthermore, irrespective of two-wavelength type or three-wavelength type, white emission can be applied also to lighting and so on. From such meaning too, development of a highly efficient white organic light-emitting element is desired.